Hard copy devices, such as printers, generally print two kinds of data. The first type of data is stroke data, usually text and graphics data. The second type of data is continuous tone ("contone") data, which is usually image data, for example, a data representation of a photographic image.
High quality rendering of stroke data requires a printer that can produce a spatial resolution of 600 dots per inch ("dpi") or more. This resolution is required, for example, to reduce jaggedness of diagonal edges or to reproduce fine seriphs in text.
Contone data, on the other hand, does not require such high spatial resolution, but requires a printer than can produce gray shades. Indeed, if the printer is capable of producing 256 gray shades (which is typical intensity resolution of image scanners), a 200-250 dpi printer will generally suffice. Such a printer provides appropriate spatial resolution for the human eye to perceive smooth shades.
Unfortunately, most existing printers are binary printers that can reproduce only two gray shades, black and white. These printers simulate more gray shades by logically combining several dots into cells and relying on the spatial integration property of the human eye to stimulate smooth shades. As an example, a 3200 dpi printer can achieve appropriate gray shades by forming cells from 16.times.16 arrays of dots, resulting in 200 cells per inch (sometimes referred to as lines per inch). By controlling the number of dots that are black (or white) in this array, each cell can represent up to 257 gray levels (0 through 256, with 256 represented by all dots being black). The spatial resolution of the human eye will perceive smooth shades with such a printer.
Thus, binary printers reproduce high quality contone data only at an extremely high resolution. To produce both contone and stroke data, therefore, the resolution is defined by the contone image requirements. Consequently, the stroke data is printed at a higher resolution than needed. Furthermore, printing at the high resolution needed for contone data with existing systems makes them expensive.
Therefore, a need has arisen for a printer that can print both stroke and contone data at the resolution of stroke data.